1. Field of the Invention
This invention relates to a method of preventing offset in electrophotography. More specifically, it relates to a method of preventing an offset phenomenon of a toner when a receptor sheet bearing a toner image on its surface is brought into contact with a hot roller to fix the toner image.
2. Description of the Prior Art
In heat fixing toner particles onto a copying paper sheet in electrophotography, the heat pressing method using a hot roller is advantageous in view of thermal efficiency, prevention of the occurrence of fire, and the fixing operation. Since an image of the toner particles come into contact with the surface of fixing rollers under heat and pressure, the toner particles partly adhere, and is transferred, to the surface of the fixing rollers (a so-called offset phenomenon), and is again transferred to the copying paper sheet to contaminate it.
To prevent such a defect, some methods have already been proposed. One typical method is to make the surface of the fixing rollers from a releasing and soil-resistant material such as a fluorine-containing resin and form a thin film of an offset preventing liquid such as a silicone oil on its surface. This method, however, has the disadvantage that the mechanism of the fixing device is complex and one has to take the trouble of using the offset preventing liquid in the fixing operation. Another typical method is to prevent the offset phenomenon by including a substance acting as a releasing agent such as a low-molecular-weight olefin resin into the toner particles, and liberate this substance in the form of a liquid on the surfaces of the particles at the time of fixing by rollers.
As will be clearly shown by an example given hereinafter, the latter method may be effective for heat fixing at relatively low temperature of less than 160.degree. C., but it has been found that at high temperatures of more than 170.degree. C., its offset preventing effect is not sufficient.
It is strongly desired to increase the temperature of a hot roller in the hot roller fixing method in order to make possible high-speed copying and to enable the use of a high softening point fixing resin which can be fixed at a high fixing temperature and therefore does not undergo blocking when used as a toner.